Optical communications – Transmitter and receiver system – Including optical waveguide
Reexamination Certificate
1999-07-23
2004-04-27
Pascal, Leslie (Department: 2633)
Optical communications
Transmitter and receiver system
Including optical waveguide
C398S159000, C398S194000, C398S195000, C398S201000, C398S209000, C398S021000
Reexamination Certificate
active
06728491
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a polarization-mode dispersion detecting method, and a dispersion compensation controlling apparatus and a dispersion compensation controlling method used when polarization-mode dispersion or chromatic dispersion of a transmission optical signal which becomes a factor of limitation on a transmission distance of a high-speed optical signal in a very high-speed optical communication system adopting, for example, optical time division multiplexing.
BACKGROUND ART
In a trunk-line optical communication system, a system with a transmission rate 10 Gb/s (gigabit/second) is in stage of practical application. On the other hand, there is a demand for a larger capacity of the optical communication system with a rapid increase of an information quantity. Considered as candidates for employable system are time division multiplexing (including optical time division multiplexing) and wavelength division multiplexing. Particularly, in time division multiplexing, a lot of researches on a very high-speed optical communication system with a transmission rate 40 Gb/s (hereinafter referred to as a 40 Gb/s optical communication system) are conducted inside and outside the country.
However, the 40 Gb/s optical communication system has a problem that a transmission distance of an optical signal is limited since a transmission waveform is deteriorated by effects of polarization-mode dispersion and chromatic dispersion. Namely, in this system transmission line, a chromatic dispersion value and a polarization-mode dispersion value are factors of limitations of a transmission rate and a transmission distance. Hereinafter, results of simulation and results of experiment on chromatic dispersion will be described with reference to
FIGS. 66 through 72
, and polarization-mode dispersion will be described with reference to
FIGS. 73 through 75
.
Although a term “dispersion” is generally used to mean “chromatic dispersion”, when merely the term “dispersion” is used hereinafter, it means both “polarization-mode dispersion” and “chromatic dispersion” unless specifically mentioned.
First, chromatic dispersion will be schematically described. Since a chromatic dispersion tolerance (tolerance means an allowance) is inversely proportional to the square of a bit rate, a chromatic dispersion tolerance of 10 Gb/s is 800 ps
m, while a chromatic dispersion tolerance of 40 Gb/s is about 50 ps
m that is one sixteenth of 800 ps
m, which is severer.
FIG. 66
shows a structure of an experimental system to evaluate dispersion compensation tolerance after 50 km transmission over a 1.3 &mgr;m zero-dispersion fiber (SMF: Single Mode Fiber) in 40 Gb/s optical time division multiplexing (OTDM: Optical Time Division Multiplexing). Here are used a chromatic dispersion value=18.6 ps
m/km, and a total dispersion value=930 ps
m. A 40 Gb/s optical transmitter
121
a
shown in
FIG. 66
is a signal light source. A signal light intensity-modulated in an intensity modulator
121
b
is inputted to a receiving side (hereinafter referred to as a receiving terminal, occasionally) over a DCF (Dispersion Compensating Fibers)
124
via the SMF
123
. On the receiving side, a preamplifier
122
a
and a 40 Gb/s optical receiver
122
b
perform a demodulating process.
FIG. 67
shows a result of an evaluation experiment in this experimental system, wherein a transverse axis represents total dispersion quantity (unit: ps
m) while a vertical axis represents power penalty (unit: dB). If here is required a power penalty 1 dB or less as an evaluation reference of the transmission line, a dispersion compensation tolerance (dispersion width) is 30 ps
m, this value corresponding to 2 km or less in transmission using SMF. Namely, when a repeater spacing, that is, a distance between stations, is not constant as in a ground system, it is necessary to optimize a dispersion compensation quantity (high-accuracy dispersion compensation of about 100%) of each repeater section.
Additionally, a chromatic dispersion value of an optical fiber transmission line changes with time with a change of laying environment such as temperature, pressure and the like. For example, in the case of a change in temperature from −50 to 100° C., a quantity of the change in dispersion of SMF
50
km is estimated to be 16 ps
m as shown by the following formula:
[
dispersion
change
quantity
]
=
[
temperature
dependency
of
zero
dispersion
wavelength
]
×
[
temperature
change
]
×
[
dispersion
slope
]
×
[
transmission
distance
]
=
0.03
(
nm
/
°
C
.
)
×
150
(
°
C
.
)
×
0.07
(
ps
/
nm
2
/
km
)
×
50
(
km
)
=
16
ps
/
nm
This value is more than a half of the dispersion tolerance 30 ps
m, which has to be considered in full in system designing. The reason is that when a temperature becomes 100° C. during system operation, he value does not meet the reference of penalty 1 dB in the worst case, even if the dispersion compensation quantity is optimized at −50° C. when the operation of the system is started. Depending on characteristics or a structure of the dispersion compensator, it is impossible to continuously set a dispersion compensation quantity, so that there is a case where the dispersion compensation quantity can be set to only a value slightly deviated from an optimum value when the operation of the system is started. In this case, the value might not meet the reference of penalty 1 dB even with a change in temperature below 150° C.
In the above consideration, in order to realize a very high-speed optical communication system above 40 Gb/s, it is necessary to first optimize dispersion equalization (dispersion compensation quantity) in each repeater section when the system operation is started, and to secondary configure “an automatic dispersion equalization (compensation) system” optimizing dispersion equalization (dispersion compensation value) correspondingly to a change with time of a transmission line dispersion value even during the system operation. Meanwhile, this automatic dispersion equalization system is required not only in the SMF transmission system but also in the case where a 1.55 &mgr;m wavelength dispersion shifted fiber (DSF: Dispersion Shifted Fiber) having a small chromatic dispersion value is used. Elemental techniques for realizing the automatic dispersion equalization system are summarized into three points, (a) through (c) below:
(a) realization of a variable dispersion equalizer (compensator);
(b) method of monitoring a chromatic dispersion value (or a total dispersion quantity after dispersion equalization [compensation]) of a transmission line; and
(c) method of controlling feedback optimization of a variable dispersion equalizer (compensator).
As a method of measuring a chromatic dispersion value of an optical fiber, there has been used a pulse method or a phase method in which light having plural different wavelengths is inputted to an optical fiber, and a group delay difference or a phase difference in the output light is measured. However, in order to always measure dispersion during the system operation using these methods, a set of chormatic dispersion measuring devices are required in each repeater section. Further, in order to measure a dispersion quantity without interrupting transmission of data signal light, it is necessary to wavelength-multiplex measuring light having a wavelength different from that of the data signal light.
Assembling the pulse method or the phase method in an optical transmission apparatus is not realistic from the points of view of size and economy. Further, when a wavelength different from that of the main signal light, there is a possibility of lacking accuracy
Akiyama Yuichi
Ishikawa George
Ooi Hiroki
LandOfFree
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